Researchers found they suffered cognitive loss similar to that seen when a person ages from 50 to 70, or when losing 10 IQ points.
Scores improved over time but experts warned it was “very possible that some of these individuals will never fully recover”.
Study author Professor David Menon, from the Division of Anaesthesia at the University of Cambridge, said: “Cognitive impairment is common to a wide range of neurological disorders, including dementia, and even routine ageing.”
“But the patterns we saw – the cognitive ‘fingerprint’ of Covid-19 – were distinct from all of these.”
The study by the University of Cambridge and Imperial College London involved 46 people treated for coronavirus at Addenbrooke’s Hospital in Cambridge.
They completed brain tests – including for memory, attention and reasoning – and their scores were compared with healthy controls.
Covid survivors were less accurate and had slower response times than those who had not been sick, with the effects strongest for patients who had needed ventilation.
The deficits were still detected six months after participants had first contracted the virus.
Professor Menon added: “We followed some patients up as late as ten months after their acute infection, so we’re able to see a very slow improvement.”
“While this was not statistically significant, it is at least heading in the right direction, but it is very possible that some of these individuals will never fully recover.”
The team noted that although their study looked at hospitalised patients, people who were not sick enough to be admitted may also have signs of mild impairment.
Professor Adam Hampshire, of Imperial, said: “Around 40,000 people have been through intensive care with Covid-19 in England alone and many more will have been very sick, but not admitted to hospital.”
“This means there is a large number of people out there still experiencing problems with cognition many months later.”
“We urgently need to look at what can be done to help these people.”
The findings were published in the journal of clinical medicine.